Colon cancer, which is also known as cancer of the large bowel and colorectal cancer, is second only to lung cancer as a cause of cancer death in the United States. Colorectal cancer is a common malignant condition that generally occurs in individuals 50 years of age or older; and the overall incidence rate of colon cancer has not changed substantially during the past 40 years. (Harrison's Principles of Internal Medicine, 14/e, McGraw-Hill Companies, New York, 1998). The treatment of colon cancer once diagnosis is made depends on the extent of the cancer's invasion of the colon tissue, lymph nodes, and metastasis to other organs such as the liver. The survival rate for patients diagnosed with early-stage cancer is about 90% survival after 5 years. The five-year survival rate drops if the cancer is not detected until the cancer has spread beyond the mucosal layer of the colon, and drops significantly further if, when detected, the cancer has spread beyond the colon to the lymph nodes and beyond. Thus, it is critical to diagnose colon cancer at the earliest possible stage to increase the likelihood of a positive prognosis and outcome.
The traditional method of colon cancer diagnosis is through the use of non-invasive or mildly invasive diagnostic tests, more invasive visual examination, and histologic examination of biopsy. Although these tests may detect colon cancers, each has drawbacks that limit its effectiveness as a diagnostic tool. One primary source of difficulty with most of the currently available methods for diagnosing colorectal cancer, is patient reluctance to submit to, or follow through with the procedures, due to the uncomfortable or perceived embarrassing nature of the tests.
Some of the less invasive diagnostic methods include fecal occult blood testing and digital rectal exam. A digital exam may detect tumors at the distal end of the colon/rectum, but is not effective at more proximal levels. The usefulness of tests for occult blood is hampered by the intermittent bleeding patterns of colon cancers, which can result in a high percentage of false negative results. For example, approximately 50 percent of patients with documented colorectal cancers have a negative fecal blood test. In addition, false-positive fecal occult blood tests may also present problems for accurate diagnosis of colon cancer, because a number of non-colon cancer conditions (e.g.: gingivitis, ulcer, or aspirin use) may yield positive test results, resulting in unnecessary invasive follow-up procedures. These limitations of the less-invasive tests for colon cancer may delay a patient's procurement of rapid diagnosis and appropriate colon cancer treatment.
Visual examination of the colon for abnormalities can be performed through endoscopic or radiographic techniques such as rigid proctosigmoidoscopy, flexible sigmoidoscopy, colonoscopy, and barium-contrast enema. These methods are expensive, and uncomfortable, and also carry with them a risk of complications.
Another method of colon cancer diagnosis is the detection of carcinoembryonic antigen (CEA) in a blood sample from a subject, which when present at high levels, may indicate the presence of advanced colon cancer. But CEA levels may also be abnormally high when no cancer is present. Thus, this test is not selective for colon cancer, which limits the test's value as an accurate and reliable diagnostic tool. In addition, elevated CEA levels are not detectable until late-stage colon cancer, when the cure rate is low, treatment options limited, and patient prognosis poor.
More effective techniques for colon cancer diagnosis and evaluation of colon cancer treatments are needed. Although available diagnostic procedures for colon cancer may be partially successful, the methods for detecting colon cancer remain unsatisfactory. There is a critical need for diagnostic tests that can detect colon cancer at its early stages, when appropriate treatment may substantially increase the likelihood of positive outcome for the patient.